Metal oxide-hydrogen batteries, such as nickel oxide-hydrogen batteries, have seen use as aircraft starter batteries and in aerospace applications, because they are rechargeable, have an extremely long cycle life and provide a uniform output during the entire discharge cycle.
In the typical metal oxide-hydrogen battery, the battery cells are sealed in an outer metal pressure vessel that contains pressurized hydrogen gas. On discharge of the battery, the hydrogen gas diffuses through the electrolyte surrounding the catalyst surfaces of the negative electrodes and becomes disassociated to the mono-atomic form. The mono-atomic hydrogen is ionized and combines with hydroxyl ions to form water, with an electron being released in the process of forming each hydrogen ion. In addition, hydroxyl ions are formed at the negative electrode by the reaction of water with the available oxygen content of the nickel oxide. As a result of these reactions, an electron current is produced in an exterior circuit.
On recharging, the reaction is reversed with the recharging being characterized by the regeneration of hydrogen at the negative electrode and the reoxidation of the nickel hydroxide at the positive electrode.
In fabricating a nickel-oxide hydrogen battery, the cell modules are placed within the shell of the pressure vessel and dome-shaped heads are then welded to each end of the shell. To facilitate welding, a metal weld ring formed of the same metal or alloy as the vessel, preferably Inconel, is located at the joint between each end of the shell and the respective head. The typical weld ring has a generally T-shaped cross section, including a base portion, which is located in contact with the inner surfaces of the shell and head, and a radial fin which extends outwardly between the opposed or abutting ends of the shell and head. Laser or electron beam welding is commonly used to connect the weld ring to the ends of the shell and head.
The weld ring provides several functions. The weld ring prevents blow-by of the molten metal during welding and prevents the molten metal from contacting the battery cells which could have an adverse effect on the performance of the battery. In addition, the weld ring serves to provide make-up metal to replace evaporated molten metal during welding, and provides strength for the heat effected zone of the weld. As a further function, the weld ring can provide a mounting frame for the cell modules to position the cell modules within the vessel and prevent shifting of the cell modules.
In order to provide these functions, the weld rings, as used in the past, have been heavy rigid structures having a relatively thick outer annular section, as well as spokes or braces which extend across the central portion of the ring.
In use, a metal oxide-hydrogen battery may be subject to thousands of charging and discharging cycles, in which the internal hydrogen pressure varies from a minimum pressure of perhaps 50 psi to a maximum pressure of about 900 psi. For satellite or aerospace applications, it is necessary to subject the metal oxide hydrogen battery to a rigorous test procedure, in which the battery is subjected to approximately 40,000 cycles at an elevated pressure about 2.5 times the maximum pressure to be encountered in service, or about 2,200 psi. Under this test procedure, it has been noted that cracking can occur at the junction between the shell and the weld. During the pressure cycling, the shell will expand radially to a greater extent than the dome-shaped head, thus subjecting a portion of the weld adjacent the shell to stress, which can cause cracking and failure. The welded joint in the metal oxide hydrogen battery cannot be heat treated due to the fact that the battery is contained within the vessel and the heat treatment would destroy the battery.
While it is possible to overcome the cracking problem at the welded joint by increasing the wall thickness of the pressure vessel, this solution is not satisfactory, for it will increase the weight of the battery which is of prime concern in a satellite or aerospace application.